Cam phasers are well known in the automotive art as elements of systems for reducing combustion formation of nitrogen oxides (NOX), reducing emission of unburned hydrocarbons, improving fuel economy, and improving engine torque at various speeds. The terms "cam phaser" and "camshaft phaser" may be used interchangeably herein. Typically, a cam phaser employs a first element driven in fixed relationship to the crankshaft and a second element adjacent to the first element and mounted to the end of the camshaft in either the engine head or block. A cam phaser is commonly disposed at the camshaft end opposite the engine flywheel, herein referred to as the "front" end of the engine. The first element is typically a cylindrical stator mounted onto a crankshaft-driven gear or pulley, the stator having a plurality of radially-disposed inwardly-extending spaced-apart lobes and an axial bore. The second element is a vaned rotor mounted to the end of the camshaft through the stator axial bore and having vanes disposed between the stator lobes to form actuation chambers therebetween such that limited relative rotational motion is possible between the stator and the rotor. Such a phaser is known in the art as a vane-type cam phaser.
The disposition of the rotor in the stator forms a first, or timing-advancing, array of chambers on first sides of the vanes and a second, or timing-retarding, array of chambers on the opposite sides of the vanes. The apparatus is provided with suitable porting so that hydraulic fluid, for example, engine oil under engine oil pump pressure, can be brought to bear controllably on opposite sides of the vanes in the advancing and retarding chambers. Control circuitry and valving, commonly a multiport spool valve, permit the programmable addition and subtraction of oil to the advance and retard chambers to cause a change in rotational phase between the stator and the rotor, in either the rotationally forward or backwards direction, and hence a change in timing between the pistons and the valves.
As an engine camshaft rotates, each eccentric cam lobe, in its tum, displaces a spring-loaded cam-following mechanism outwards from the axis of the camshaft to open its dedicated valve, then allows return motion of the mechanism to close the valve. Because of friction, more energy is expended in opening each valve than is recovered in closing each valve. Within a cam phaser, this energy imbalance is expressed as a torsional bias in the opening-retarding direction. That is, the operating equilibrium position of the rotor within the phaser appears biased in the retarding direction with reference to an anticipated position based solely on hydraulic considerations. This means that the hydraulic system driving the rotation of the rotor, in responding to a demand for advancing of the valve timing, must overcome not only the inertia of the system but also the retarding bias of the camshaft friction.
What is needed is a mechanical means in a vane-type cam phaser for offsetting or neutralizing the retarding frictional bias of the camshaft to permit compensated hydraulic operation of the phaser and, consequently, more rapid opening-advancing response of the phaser.